A Wide Area Network (WAN) is a large computer network that usually spans a large geographical area, such as a campus, city, country, and even world. A wired or wireless infrastructure or optical fiber connections may be used for connecting to a WAN. Technologies that may be used in WANs are ATM (Asynchronous Transfer Mode), FDDI (Fiber-Distributed Data Interface) and SMDS (Switched Multi-megabit Data Services), although these older technologies are being replaced by Ethernet-based technologies, e.g. Metro Ethernet.
Metro Ethernet is accordingly a computer network based on the Ethernet standard covering a wide area. Frequently, a metro Ethernet connects subscribers and businesses to a higher layer service such as the Internet.
A typical service provider metro Ethernet network is a collection of Layer 2 or 3 switches or routers connected through wireless infrastructure or optical cable, with the metro Ethernet network typically having a ring, hub-and-spoke, full mesh or partial mesh topology.
DiffServ or differentiated services is a method of providing quality of service on large networks such as MANs. DiffServ deals with bulk flows of data rather than single flows and single reservations. Typically a single negotiation will be made for the transfer of packets from a single customer or subscriber. Service level agreements (SLAs) are the contracts which specify what classes of traffic (classes of service) will be provided by a service provider, the guarantees needed for each class, and the amount of data to be sent for each class.
When data packets enter a DiffServ network, the attached device, e.g., host or router, classifies the data packet and sets the DSCP accordingly. For example, the customer edge router may set the “type of service” or “class of service” field, typically called the Differentiated Service Code Point or DSCP in the IP header according to the class of the data, with better classes typically receiving higher numbers.